1. Field of the Invention
This invention relates to a secondary battery, particularly, to a secondary battery which suppresses the generation of dendritic deposition in lithium metal or zinc metal during repetition of charging/discharging.
2. Description of the Related Art
Recently, an increase in the earth's temperature due to the greenhouse effect caused by an increase in the amount of CO.sub.2 gas included in the atmosphere has been predicted. Since thermal power plants, which burn fossil fuel and convert obtained thermal energy into electric energy, discharge a large amount of CO.sub.2 gas, it becomes more and more difficult to construct new thermal power plants. Accordingly, in order to effectively utilize electric power generated by generators, such as thermal power plants and the like, so-called load leveling has been proposed in which night power, serving as dump power, is stored in secondary batteries installed in general houses and is used during day time where electric power consumption is large to level the load.
In the field of electric vehicles having the feature of not emitting substances causing air pollution, such as COx, NOx, hydrocarbons and the like, the development of high-energy-density secondary batteries has been expected. In the field of power supplies for portable devices, such as book-size personal computers, word processors, video cameras, portable telephone sets and the like, the development of small, light and high-performance secondary batteries has been urgently requested.
As such small, light and high-performance secondary batteries, so-called "lithium-ion batteries" of a rocking-chair type, which use a lithium-intercalation compound in which lithium ions are deintercalated from between layers in reaction during charging as a cathode (a positive electrode) material, and a carbonous material such as graphite in which lithium ions can be intercalated between layers of a plane having the shape of a six-membered-ring network formed by carbon atoms as an anode (a negative electrode) material, have been developed and partially put into practical use.
In the "lithium-ion battery", however, since the anode made of a carbonous material can theoretically intercalate only 1/6 lithium atom per carbon atom at most, a high-energy-density secondary battery which utilizes with a lithium primary battery using metallic lithium as the anode material has not yet been realized. If it is intended to store a greater amount of lithium than an amount of lithium which can be intercalated between carbon layers of the anode during charging, dendritic lithium metal grows on the surface of the carbonous material to cause short circuit. Accordingly, a high-capacity secondary battery which stores a greater amount of lithium than an amount of lithium capable of being intercalated between layers has not yet been realized.
In high-capacity lithium batteries using metallic lithium as the anode, lithium dendrite which is a principal cause for internal-shorts is apt to be generated by repetition of charging/discharging. Since it is not easy to suppress the growth of such lithium dendrite, it is difficult to provide lithium secondary batteries.
Although lithium secondary batteries using a metal, such as lithium metal, aluminum or the like, as the anode, in which lithium metal is deposited during charging and high energy density can be expected, have been studied, such batteries do not yet have a sufficient life so as to be practically used.
In secondary batteries using a carbonous material for the anode or secondary batteries using an anode where lithium metal is deposited during charging, when lithium dendrite grows to provide a state of internal-shorts between the anode and the cathode, the battery is heated by consumption of the energy of the battery during a short time period, and the solvent of the electrolyte solution is thereby decomposed to generate a gas and increase the internal pressure, thereby, in some cases, damaging the battery. In order to secure safety for lithium secondary batteries including the above-described lithium ion batteries (secondary batteries which utilize the oxidation-reduction reaction of lithium ions will be hereinafter generically termed "lithium secondary batteries"), an attempt has been made in which a porous film made of polyethylene or polypropylene having a melting point of 120.degree. C.-170.degree. C. is used for the separator between the cathode and the anode, and the separator melts when the internal temperature rises to the melting point of the material due to internal-shorts of the battery caused by some reason, to fill pores and thereby to electrically insulate the anode from the cathode and stop the battery reaction. Although this approach functions as a safety precaution for an emergency, this is not a complete solution for drastically increasing the life of the anode of a lithium secondary battery.
In the above-described lithium secondary batteries using a porous film made of an organic polymer as the separator, the battery operates only when the temperature is equal to or higher than 120.degree. C., and the function of the battery having the separator whose pores have been filled with the melted organic polymer does not recover even if the temperature decreases because the state of insulation between the cathode and the anode remains. Accordingly, it is desired to develop means for realizing a high battery capacity, and suppressing the growth of lithium dendrite during charging and thereby increasing the cycle life of the battery.
In secondary batteries such as nickel-zinc batteries or air-zinc batteries, as in lithium secondary batteries, the dendritic deposition of zinc metal is liable to be produced due to the repetition of charging/discharging and threads through the separator to cause internal-shorts between the zinc anode and the cathode, thereby shortening the cycle life.
Accordingly, in lithium secondary batteries and zinc secondary batteries, it is strongly desired to develop a battery having a sufficient cycle life and high capacity.